The present invention relates to an improved aqueous ink for ink-jet printing.
Generally, ink for use in ink-jet printing should satisfy several conditions in order to yield an excellent print.
First, in order that the ink be compatible with ink droplet formation and control of the direction of the ejected ink droplet streams, it is necessary that its viscosity, surface tension, specific electric conductivity, and density each fall within certain appropriate ranges. It is necessary that no precipitates be separated, due to chemical change or other causes, from the ink during an extended period of storage, or during the periods when the apparatus is not in use. Nor should the physical properties of the ink otherwise change during the above-mentioned periods.
The diameter of each ink-jet nozzle in a conventional ink-jet printing apparatus is usually in the range of 10 microns to 60 microns, and, if the nozzles are plugged, for instance, with precipitates separated from the ink, it becomes impossible to eject the ink droplets from the nozzles. Even if the nozzles are not completely plugged, if solid components or viscous materials in the ink become affixed around the nozzle, or if the physical properties of the ink change, deviating from the predetermined physical properties achieved at the time of the preparation, the desired printing quality, the desired ink ejection stability, and the desired ink ejection response cannot be obtained.
Second, it is desired that the ink provide adequately high contrast and clearness in the printed image. If, however, the percentage of a dye contained in the ink is increased in an effort to increase the image contrast, the nozzles become easily plugged with the ink. Under such circumstances, a dye which is highly soluble in the solvent used in the ink, and which has a high extinction coefficient, is desired.
Third, it is necessary that the image printed by the ink dry rapidly. In a conventional ink for ink-jet printing, in order to prevent the ink from drying within the nozzle when printing is not actually taking place, a comparatively large amount of a wetting agent is contained in the ink, and, as a result, in order to speed up the drying of the printed images, a special paper with high ink-absorption capacity is used. This not only results in a substantial spreading of the image, but precludes the use of many conventional types of paper.
Fourth, it is necessary that the ink for ink-jet printing be such that the printed images are resistant to water, light and physical wear.
So far, many proposals have been made for an ink for ink-jet printing, but from the viewpoint of practical use, a satisfactory ink which is capable of meeting the above-described requirements has not been found.